Capacitive pressure sensor with intrinsic temperature compensation

ABSTRACT

Pressure sensors and their methods of use are described. In one embodiment, a pressure sensor includes a probe body and a capacitive sensor disposed at a distal end of the probe body. The capacitive sensor produces a sensing capacitance. The pressure sensor also includes a shunt capacitance. In the described pressure sensor, a change in the sensing capacitance due to dimensional changes associated with a temperature change is offset by a corresponding change in the shunt capacitance.

FIELD

The disclosed embodiments are generally directed to capacitive-basedpressure sensors.

BACKGROUND

Extrusion and injection molding of plastics is extensively used in theproduction of components for numerous industries and applications.During the manufacturing of these various components, it is important tomeasure the pressure and temperature of the melt within the system toensure accurate and reproducible component production. If the meltpressure is too low, insufficient mold packing, short shots, and otherundesirable manufacturing defects may occur. Further, if the moldpressure is too high, it may result in excessive flash, materialejection, and possible equipment malfunction. Thus, pressure sensors aregenerally incorporated into one or more locations within an extruder, orinjection molding system, to monitor the extrusion, or injection,process. However, polymer melt temperatures range up to 400° C., ormore, depending upon the particular polymer. Therefore, pressure sensorsappropriate for use in high temperature environments are typically usedfor monitoring these systems. In addition, high temperature pressuresensors are also used in various other applications including, but notlimited to, pressure monitoring of turbine engines, oil drilling, foodprocessing operations, and other appropriate applications.

SUMMARY

In one embodiment, a pressure sensor includes a probe body comprising apressure deflectable diaphragm end formed of a first material having afirst coefficient of thermal expansion. The diaphragm end includes afirst surface at least partially defining a first portion of acapacitor. The pressure sensor also includes a relatively non-deformablecomponent formed of a second material having a second coefficient ofthermal expansion. The relatively non-deformable component includes asecond surface spaced from the first surface. The second surface also atleast partially defines a second portion of the capacitor. Anintermediate component is disposed at a peripheral region isolating thepressure deflectable diaphragm end and the relatively non-deformablecomponent from each other. The intermediate component is formed of athird material having a third coefficient of thermal expansion. Thethird coefficient of thermal expansion is less than the firstcoefficient of thermal expansion.

In another embodiment, a pressure sensor includes a probe body and apressure deflectable diaphragm end formed of a conductive material andcoupled to the probe body. The conductive material has a firstcoefficient of thermal expansion. Further, the pressure deflectablediaphragm end has a first surface defining at least a first portion of acapacitor. The pressure sensor also includes an alumina disk with asecond coefficient of thermal expansion that is less than the firstcoefficient of thermal expansion. The disk includes a metallizationlayer. The metallization layer is spaced from the first surface suchthat the metallization layer at least partially defines a second portionof the capacitor. A spring is disposed between the probe body and thealumina disc. The spring biases the alumina disk toward the pressuredeflectable diaphragm end. An annular shim component is disposed at aperipheral region of the alumina disk to isolate the alumina disk fromthe pressure deflectable diaphragm end. The annular shim is formed of athird material with a third coefficient of thermal expansion. The thirdcoefficient of thermal expansion is less than the first coefficient ofthermal expansion.

In yet another embodiment, a pressure sensor includes an intermediatecircuit that produces an electrical voltage signal proportional to adifference between a reference capacitance and a sensed capacitance.

In another embodiment, a pressure sensor comprises a probe assemblyincluding: a relatively rigid body; a distal end having a capacitivepressure sensor, the capacitive pressure sensor capable of producing asensed capacitance as a result of pressure acting on the capacitivepressure sensor; and a proximal end opposite the distal end. Anintermediate circuit enclosure is disposed at the distal end. Acapacitive detection bridge circuit is housed within the intermediatecircuit enclosure. The capacitive detection bridge circuit produces anelectrical voltage signal proportional to a difference between areference capacitance and the sensed capacitive signal. The pressuresensor also includes a remote circuit enclosure. A relatively flexibleinterconnect couples the remote circuit enclosure to the intermediatecircuit enclosure. A main circuit is disposed in the remote circuitenclosure. The remote circuit enclosure is connected to the intermediatecircuit enclosure by the relatively flexible interconnect. Thecapacitive detection bridge circuit transmits the electrical voltagesignal to the main electrical circuit through the interconnect.

In one embodiment, a pressure sensor includes a probe body and acapacitive sensor disposed at a distal end of the probe body. Thecapacitive sensor produces a sensing capacitance. The pressure sensorfurther includes a shunt capacitance, wherein a change in the sensingcapacitance resulting from a change in temperature is offset by acorresponding change in the shunt capacitance.

In another embodiment, a method of making a pressure sensor includes:arranging a pressure deflectable diaphragm cap having a first capacitivesurface on a probe body; electrically coupling the first capacitivesurface to the probe body; selecting a material having a desireddielectric constant and forming a non-deformable component from thematerial; forming a second capacitive surface on a portion of thenon-deformable component; connecting a lead to the second capacitivesurface and positioning the non-deformable component within the probebody wherein a shunt capacitance is defined between the lead and theprobe body; and arranging the non-deformable component relative to thepressure deflectable diaphragm cap by spacing the second capacitivesurface away from the first capacitive surface such that the first andsecond surfaces define a capacitive sensor having a sensing capacitance,wherein a change in the sensing capacitance resulting from a change intemperature is offset by a corresponding change in the shuntcapacitance.

In one embodiment, a pressure sensor includes a tubular probe bodyhaving a proximal end and a distal end. The pressure sensor alsoincludes a capacitive sensor disposed at the distal end of the probebody. A lead is electrically coupled to the capacitive sensor andextends along an interior space of the tubular probe body toward theproximal end. At least one support is formed of a material having arelatively low dielectric constant and disposed within the tubular probebody. The at least one support is constructed and arranged to supportthe lead within the tubular probe body and space the lead away from aninner wall of the tubular probe body.

In another embodiment, a pressure sensor includes a tubular probe bodyhaving a proximal end and a distal end. The tubular probe body includesa channel formed in a wall of the tubular probe body and extending fromthe distal end to the proximal end. A capacitive sensor is disposed atthe distal end of the probe body. A lead is electrically coupled to thecapacitive sensor and extends along an interior space of the tubularprobe body toward the proximal end. A temperature sensor is disposed atthe distal end of the tubular probe body, and a temperature sensor leadis disposed in the channel and connected to the temperature sensor.

It should be appreciated that the foregoing concepts, and additionalconcepts discussed below, may be arranged in any suitable combination,as the present disclosure is not limited in this respect.

The foregoing and other aspects, embodiments, and features of thepresent teachings can be more fully understood from the followingdescription in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF DRAWINGS

The accompanying drawings are not intended to be drawn to scale. Forpurposes of clarity, not every component may be labeled in everydrawing. In the drawings:

FIG. 1 is a schematic representation of a pressure sensor incorporatingan intermediate circuit;

FIG. 2 is a schematic representation of a pressure sensor incorporatingan intermediate circuit and flexible interconnect;

FIG. 3 is a schematic cross-sectional view of the pressure sensor takenalong line 3-3 of FIG. 1 of an intermediate circuit;

FIG. 3A is an enlarged view of the distal end encircled by line 3A ofthe pressure sensor depicted in FIG. 3;

FIG. 4 is a schematic cross-sectional view of a portion of a section ofthe probe body of the pressure sensor;

FIG. 5 is a schematic end view of a pressure sensor including atemperature sensor lead channel;

FIG. 5A is a cross-sectional view of the pressure sensor depicted inFIG. 5;

FIG. 5B is a cross-sectional view of the pressure sensor depicted inFIG. 5A including a temperature sensor lead groove;

FIG. 5C is a cross-sectional view of a pressure sensor including atemperature sensor lead bore;

FIG. 6 is a schematic cross-sectional view of a temperature sensordisposed in a temperature sensor channel in the probe body of thepressure sensor;

FIG. 7 is a graph depicting the zero output and span change of thecapacitive pressure sensor between two different temperatures;

FIG. 8 is a schematic representation of many of the capacitancesassociated with the pressure sensor;

FIG. 9 is a schematic representation of the distal end of the pressuresensor;

FIG. 10 is a graph showing changes in the gap versus temperature;

FIG. 11 is a graph of percent zero output change from 25° C. to 350° C.versus changes in component dimension “L” from FIG. 9;

FIG. 12 is a graph of output spans for the same pressure sensor atdifferent temperatures with different initial capacitor gaps “g” fromFIG. 9; and

FIG. 13 is a schematic representation of a portion of an exemplarypressure sensor.

DETAILED DESCRIPTION

Pressure sensors for monitoring the pressures associated with a polymermelt during extrusion and injection molding processes are subjected toelevated temperatures ranging up to 400° C., or more, depending upon theparticular polymer. The inventors have recognized that exposure to theselarge temperature ranges not only requires correction of the outputtemperature signal due to high temperature effects, but thesetemperatures also have a detrimental effect on component life.Therefore, the inventors have recognized a need to provide a hightemperature pressure sensor probe capable of accurately measuring thepressure while also providing increased reliability and lifetime.Further, in order to provide higher fidelity signals with greatersensitivity, the inventors have also recognized the need to reduceoverall signal losses and sources of interference within the pressuresensor. Additionally, the inventors have recognized a need to optimizethe change in capacitance due to pressure while minimizing andcontrolling the overall capacitance of the pressure sensor.

The inventors have recognized that the lead between the capacitivepressure sensor and proximately located main circuit is subject to straycapacitances which interfere with the sensed pressure signal. Further,the signal is subject to greater disturbances from these straycapacitances prior to conversion to a DC signal from the as measured ACcapacitance signal. Therefore, it is desirable to transform thecapacitive pressure signal into a DC signal as close to the capacitivepressure sensor as possible. However, the location of the circuitry islimited due to the high temperatures present at the distal end of theprobe body during use. The inventors have recognized that by providingan intermediate circuit capable of continuously operating at highertemperatures, it can be used to transform the pressure sensor signalfrom an AC to DC signal at a point closer to the distally locatedpressure sensor, so that the output pressure sensor signal is lesssusceptible to the influence of stray capacitances, resulting in ahigher signal fidelity, with the pressure sensor being capable ofcontinuous high temperature operation.

The inventors have also recognized that components of the distallylocated capacitive pressure sensor are subjected to thermally inducedstresses at elevated operating temperatures due to mismatches betweenthe coefficients of thermal expansion of the various components. Thesethermally induced stresses are of particular concern when they areinduced in brittle components such as ceramic components. As such, anintermediate component, and/or coating, is provided between thecomponents of the pressure sensor that have differing coefficients ofthermal expansion to reduce the thermally induced stresses and preventfatigue and/or cracking of the components. One way in which theintermediate components, and/or coatings, reduce the thermally inducedstresses between the components is to use a material having acoefficient of thermal expansion that is less than that of the materialwith the higher coefficient of thermal expansion. Since the intermediatecomponent, and/or coating, has an intermediate coefficient of thermalexpansion, each component will exhibit a reduced stress due to the moregradual transition between components with different coefficients ofthermal expansion. As an alternative, or in addition, to the above, theintermediate component, and/or coating, can include low coefficient offriction materials to reduce the thermally induced stresses transferredbetween the components due to high friction otherwise generated at theirinterfaces.

In addition to addressing high temperature operating issues associatedwith sensing the pressure, the inventors have recognized that it is alsodesirable to further reduce any stray capacitances within the system toimprove the fidelity of the output pressure signal. As such, it isdesirable to minimize shunt capacitances between the lead outputting thepressure signal and the various components forming the pressure probe.For example, since the shunt capacitance increases the closer the outputlead is to the walls of the probe body, it is desirable toconcentrically position the output lead within the probe body andminimize the lateral displacement of the output lead within the probebody that may occur due to lateral vibrations of the lead. In oneembodiment, the lead is supported at predefined spacings using a bushingand/or supports made from a material with a low dielectric constantand/or low coefficient of thermal expansion to minimize the staticand/or dynamic deflections of the lead relative to the walls of theprobe body. In some embodiments, the material has both a low dielectricconstant and low coefficient of thermal expansion. Other sources ofshunt capacitance include, but are not limited to, the output leadinteracting with other components located within the probe body such asthe leads of a temperature sensor for monitoring the processtemperature. In such an embodiment the shunt capacitance is proportionalto the distance between the output lead and the leads of the temperaturesensor. Thus, it is desirable to position the leads of sensingcomponents such as the temperature sensor as far from the output lead aspossible to reduce the magnitude of the capacitance between thosecomponents. In one embodiment, the leads of the temperature sensor, orother component, are located in a channel or groove formed within thewalls of the probe body itself to distance that component from theoutput lead and improve the fidelity of the output signal.

While it is possible to reduce the interference induced in the outputpressure signal by components due to stray capacitances (from, forexample, a temperature sensor) within the system, the inventors haverecognized that in some embodiments the interference induced by atemperature sensor can be avoided by eliminating the need for thattemperature sensor altogether. However, temperature sensors aregenerally used to help compensate for changes in the zero output andspan of the pressure signal due to temperature variations. Consequently,the inventors have recognized that it would be beneficial to provide apressure sensor that is capable of intrinsically compensating for one,or both, of the changes in the zero output and span of the pressuresignal as a result of temperature variations. The thermally inducedchanges (e.g. decrease) in the zero output can be compensated for byhaving a corresponding opposing change (e.g. increase) in the shuntcapacitances within the probe. More specifically, by controllingcomponent geometries and selecting materials with specific thermalcoefficients of dielectric constant, the rate at which the shuntcapacitance changes with temperature can be matched to offset thethermally induced changes in the zero output of pressure sensor. Inaddition to compensating for the zero output, the change in the spanrelative to an increase in temperature is also compensated for bycontrolling the rate at which the gap between the capacitive surfaces ofthe sensor increases versus temperature. Similar to the above, the rateat which the gap increases versus temperature is determined bycontrolling the component geometries and selecting materials withspecific coefficients of thermal expansion. Therefore, the pressuresignal can be compensated for temperature variations without the needfor a temperature sensor. Consequently, in embodiments not requiringtemperature sensing capabilities (as may be required by an end user orother appropriate use), the temperature sensor can be eliminated fromthe pressure sensing probe resulting in reduced complexity andadditionally reduced stray capacitances within the probe body and acorresponding increase in signal fidelity.

For the sake of clarity, the current disclosure describes a hightemperature pressure sensor for use in sensing pressures for a hightemperature polymer melt. However, it should be understood that thecurrent disclosure is not limited to use with only high temperaturepolymer melts. Instead, the high temperature pressure sensor embodimentsdisclosed herein are capable of being used in any number of differenthigh and low temperature pressure sensing applications.

Turning now to the figures, FIG. 1 depicts an embodiment of a pressuresensor probe assembly 100 arranged to isolate the main circuitry of theassembly from the heat of the melt, or other material, being monitored.Probe assembly 100 includes a probe body having a distal end including apressure sensor 102. In the present embodiment, pressure sensor 102 is acapacitive pressure sensor incorporating a pressure deflectablediaphragm coupled to a capacitive detection bridge circuit. In someinstances the bridge circuit is a diode bridge circuit such as thatdisclosed in U.S. Pat. Nos. 3,883,812 and 3,869,676, the disclosures ofwhich are incorporated herein by reference in their entireties. Theprobe assembly 100 is attached to the system being monitored through anyappropriate means including a threaded connection 104. Due to spaceconstraints within rigid probe body 106 as well as the high temperaturespresent at the distal portion of the rigid probe body during operation,aside from the capacitive sensor itself, the sensor electronics are notlocated within the probe body. Therefore, the signal output from thedistally located capacitive pressure sensor is an unamplified AC signal.This unamplified AC signal is easily disturbed due to interference fromthe relatively large stray capacitances present within the pressuresensor probe. Therefore, it is desirable to either shield or amplify thesignal to increase signal fidelity. While it is possible to shield thelead outputting the signal, in some alternative embodiments, anintermediate circuit capable of high temperature operation is locatedwithin an intermediate circuit enclosure distanced from the distal endof the probe. In some instances the intermediate circuit enclosure islocated at the proximal end of the probe. This intermediate circuit isused to amplify and/or transform the signal prior to transmitting thesignal to a remotely located main circuit contained within a maincircuit enclosure 110. In some embodiments the majority of the probecircuitry and processing of the signal is conducted within the maincircuit. Therefore, in at least some embodiments, the intermediatecircuit only includes the minimal amount of circuitry capable ofperforming the desired operation on the output signal prior totransmitting it to the main circuit for further processing.

In embodiments where the intermediate circuit enclosure is at atemperature greater than the maximum continuous operating temperature ofthe main circuit, it is desirable to distance the main circuit from theintermediate circuit enclosure. Therefore, in some embodiments, asdepicted in FIG. 2, a pressure sensor probe assembly 120 includes aninterconnect 122 to distance the main circuit from the intermediatecircuit. Interconnect 122 transmits the pressure signal output from theintermediate circuit to the main circuit. In some cases, interconnect122 is a flexible interconnect such as a flex circuit or cable capableof transmitting the output pressure sensor signal from the intermediatecircuit to the remotely located main circuit. Depending upon thedesigned operating temperature for the intermediate circuit enclosure,the main circuit enclosure can be located at any appropriate distancefrom the intermediate circuit enclosure to ensure the appropriatecontinuous operating of the circuitry contained therein without beingnegatively affected by high temperatures. For example, the distancebetween the intermediate circuit enclosure and the main circuitenclosure can be at least double, or at least quadruple, a distancebetween the intermediate circuit enclosure and the distal end of thepressure sensor probe assembly. Alternatively, or in addition to,locating the main circuit remotely, the main circuit enclosure andintermediate circuit enclosures may include active and/or passivecooling to maintain the circuitry contained therein at the appropriateoperating temperatures.

As shown in FIG. 3, pressure sensor probe assembly 200 includescapacitive pressure sensor 202 at a distal end thereof. A capacitivedetection bridge circuit can be coupled to the pressure sensor to detectthe applied pressure and output a corresponding electrical voltagesignal related to the applied pressure. The signal from capacitivepressure sensor 202 is output via lead 204 through a rigid probe body212. As depicted in the figure, rigid probe body 212 is tubular in shapethough other shapes are also possible. Lead 204 is supported by bushing206 captured within the bore of the rigid probe body. In someembodiments, bushing 206 is a thin quartz disk, that is arranged andadapted to support the lead concentrically within the rigid probe body212. After passing through bushing 206, lead 204 is transmittinglycoupled to intermediate circuit 210 located in intermediate circuitenclosure 208. As depicted in the figure, intermediate circuit 210includes a printed circuit board though any other appropriateconstruction could be used. As noted above, the pressure sensor signalis output to the intermediate circuit prior to being transmitted to theremotely located main circuit.

In some embodiments, the intermediate circuit is adapted and configuredto continuously operate at a temperature greater than the main circuit.Therefore, in such an embodiment, the intermediate circuit enclosurecontaining the intermediate circuit is advantageously located at adistance from the distal end of the probe assembly that corresponds tothe maximum continuous operating temperature of the intermediatecircuit. Alternatively, the intermediate circuit enclosure could belocated at greater distances corresponding to operating temperaturesless than the maximum continuous operating temperature of theintermediate circuit. In one exemplary embodiment, the maximumcontinuous operating temperature of the intermediate circuit isapproximately 150° C. In one embodiment, the intermediate circuit can becontinuously operated at temperatures exceeding approximately 125° C.and less than approximately 150° C. While a specific operatingtemperature range for the intermediate circuit is described the currentdisclosure is not limited to any particular temperature range. Forexample, in some embodiments, the circuit is constructed to operate attemperatures less than approximately 250° C. In such an embodiment, theintermediate circuit could be continuously operated at temperatures lessthan approximately 250° C. or any other appropriate temperature at whichthe circuit is capable of continuously operating.

Without wishing to be bound by theory, it is noted that the capacitivepressure sensor is remote from the intermediate circuit and the outputcapacitance signal is susceptible to the stray capacitances presentalong the signal transmission path until the output capacitance signalis processed by the intermediate circuit. Therefore, in someembodiments, the intermediate circuit advantageously includes a diodebridge circuit to convert the AC capacitance signal to a DC outputsignal for subsequent transmission to the remotely located main circuit.In other embodiments, the intermediate circuit also includes circuitryto amplify the signal. The remaining electronics such as, for example,the oscillator circuitry, output signal conditioning circuitry,excitation circuitry, and additional circuitry needed to provide thefinal conditioned signal for outputting to a user (or interface) arelocated in the main circuit in the remote circuit enclosure.

In some embodiments, the pressure sensor probe and the correspondingintermediate circuit are exposed to a range of operating temperatures.Therefore, in such an embodiment, it is advantageous to provide activetemperature compensation for the signal output from the intermediatecircuit due to signal errors introduced by operating the intermediatecircuit at different temperatures. Therefore, in one embodiment, atemperature sensor is provided to measure a temperature of theintermediate circuit enclosure. The measured temperature is subsequentlyoutput to the main circuit for use in correcting the output signal fortemperature effects at the intermediate circuit. Alternatively, in someembodiments, circuitry capable of correcting the output signal fortemperature effects at the intermediate circuit is included in theintermediate circuit. Depending on the particular embodiment, theintermediate circuit and associated temperature sensor are disposed on aprinted circuit board. Alternatively, the intermediate circuit couldinclude a temperature sensing circuit formed therein as the currentdisclosure is not limited to any particular temperature sensor.Regardless of the specific configuration, in the above embodiments, thepressure sensor monitors the temperature of the intermediate circuit andcorrects the output signal for any temperature effects.

FIG. 3A shows an enlarged view of the capacitive pressure sensor 202.The capacitive pressure sensor includes a pressure deflectable diaphragmcap 250 located on the distal end of the probe body 212. The distal endof the pressure deflectable diaphragm cap 250 includes a pressuredeflectable diaphragm 250 a. The pressure deflectable diaphragm 250 a ismade from a material suitable to act as an electrode to form the firstportion of the capacitive sensor. Alternatively, in embodiments wherethe pressure deflectable diaphragm 250 a is made from a material thatdoes not function as a capacitive surface, a metallization layer may bedeposited onto the interior surface of the pressure deflectablediaphragm 250 a to function as the capacitive surface. Additionally, theexterior surface of pressure deflectable diaphragm 250 a can include acoating, passivation layer, anodized layer, or other appropriate layerto provide a desired abrasion, corrosion, friction, or other desiredproperty to the pressure deflectable diaphragm exterior. The pressuredeflectable diaphragm 250 a is also electrically coupled to the rigidprobe body 212 such that it is electrically and operatively coupled withthe intermediate circuit 210 to drive the capacitive pressure sensingcircuit. In addition to the pressure deflectable diaphragm, a relativelynondeformable component, such as ceramic disk 252, includes an electrode254 on a surface oriented towards and spaced from the pressuredeflectable diaphragm 250 a to form the second portion of the capacitivesensor. Ceramic disk 252 supports and electrically insulates theelectrode 254 from the probe body. Electrode 254 can be formed on, oradhered to, the surface of ceramic disk 252 facing the pressuredeflectable diaphragm 250 a in any appropriate way. For example, in oneembodiment, electrode 254 is a plated metallic layer deposited on thesurface of ceramic disk 252. Alternatively, a separately formedelectrode could be bonded onto the surface as the current disclosure isnot limited to the way in which the electrode is formed. To provide thedesired pressure sensor output, electrode 254 on ceramic disk 252 isspaced from pressure deflectable diaphragm 250 a by a predetermined gap256. Due to use of the pressure sensor at elevated, as well as variable,temperatures it is desirable to manufacture the pressure sensorcomponents from materials having compatible coefficients of thermalexpansion to avoid thermally induced stresses. In instances where it isnot possible to match the coefficients of thermal expansion, otherdesign strategies can be employed to mitigate thermally induced stressesas discussed in more detail below.

While the above described components can be made from any appropriatecombination of materials, in one embodiment, the components aremanufactured from the following materials. The pressure deflectablediaphragm cap 250 and pressure deflectable diaphragm 250 a are made froma nickel based superalloy such as UNS N07718. UNS N07718 has acoefficient of thermal expansion of approximately 14.0×10⁻⁶/° C. at areference temperature of approximately 400° C. The correspondingrelatively nondeformable component embodied by ceramic disk 252 is ahigh alumina content ceramic, for example, a 99.5% or greateralumina-based ceramic. A ceramic comprising 99.5% alumina has acoefficient of thermal expansion of approximately 7.0×10⁻⁶/° C. at areference temperature of approximately 300° C. and a dielectric constantof 9.8+150 ppm/° C. Bushing 206 is made from quartz. Quartz has acoefficient of thermal expansion of approximately 5.5×10⁻⁷/° C.(0.55×10⁻⁶/° C.) at a reference temperature of approximately 350° C. anda dielectric constant of 3.8+28 ppm/° C. The intermediate component ismade from Fe/Ni/Co alloys such as UNS K94610 and UNS N19909 or Titaniumalloys such as Ti-6242. UNS K94610 has a coefficient of thermalexpansion of approximately 5.3×10⁻⁶/° C. at a reference temperature ofapproximately 400° C. UNS N19909 has a coefficient of thermal expansionof approximately 7.7×10″⁶/° C. at a reference temperature ofapproximately 400° C. Ti-6242 has a coefficient of thermal expansion ofapproximately 9.2×10⁻⁶/° C. at a reference temperature of approximately315° C. The probe body is made from 17-4 stainless steel.

The capacitive pressures sensor comprising the pressure deflectablediaphragm 250 a and electrode 254 are electrically coupled to theintermediate circuit by the rigid probe body and a lead 266. Theelectrode 254 is electrically coupled to the lead via lead pin 264. Insome embodiments, the lead 266 and/or lead pin 264 comprise a tubularshape to increase the flexural stiffness and the corresponding lateralvibration frequencies of those components. However, in otherembodiments, the lead 266 and/or lead pin 264 comprise a solid wire. Dueto the relative polarizations of the lead and rigid probe body it isdesirable to either shield or minimize the shunt capacitance betweenthese components. In one embodiment, the shunt capacitance is minimizedby concentrically arranging lead pin 264 and lead 266 within the rigidprobe body 212 to form an annular gap between the lead and the wall ofthe rigid probe body. As described in more detail below, the annular gapcorresponds to an air gap which provides electrical isolation andminimizes the shunt capacitance between the lead/lead pin relative tothe wall of the rigid probe body. In another embodiment, the lead/leadpin are shielded from the rigid probe body using a semi-rigid coaxialcable located within the rigid probe body interior bore.

As previously noted, the pressure sensor probe is used in a variabletemperature environment. Due to differences in the coefficients ofthermal expansion of the various components within the pressure sensorprobe, thermally induced stresses may be present. More specifically,thermally induced stresses are present at the interface between theceramic disk and the pressure deflectable diaphragm cap due to thedifferent coefficients of thermal expansion between these componentsleading to expansion and contraction of these components relative toeach other as the temperature changes. Depending upon the specificconstruction and arrangement of the components, this can lead toshifting of components relative to each other as well as possiblefatigue and cracking of the components.

If the ceramic disk shifts its axial position relative to the pressuredeflectable diaphragm, the gap 256 will have a corresponding changeresulting in a change in the output pressure sensor signal. One way inwhich to mitigate the ceramic disk from shifting relative to thepressure deflectable diaphragm, is to provide a spring 258 that axiallybiases the ceramic disk 252 towards the pressure deflectable diaphragm250 to ensure that the ceramic disk remains seated at the bottom of thepressure deflectable diaphragm counterbore 260 over the entire operatingtemperature range (e.g. −40 to 400° C.). As depicted in the figure,spring 258 is a C-ring spring that is slightly compressed in theassembled system such that it applies an axially oriented force to aproximal surface of the ceramic disk to bias at towards the pressuredeflectable diaphragm. While a C-ring spring has been depicted, anyappropriate spring could be used. The axial stiffness of the spring isselected to provide a relatively constant force to the ceramic diskduring thermal expansion and contraction. Without wishing to be bound bytheory, in some embodiments, the spring is constructed and arranged toavoid applying any lateral forces to the ceramic disk to help mitigatethe generation of any radial tensile forces therein. To avoid hightemperature deflection setting of the spring during long-term operation,it is desirable to provide a high temperature spring alloy such as, UNSNO7718 Nickel based superalloys. While spring 258 is depicted as aseparate component, in some embodiments, spring 258 is integrated intothe rigid probe body as a flexible feature that retains, and applies aforce to, ceramic disk 252.

In addition to creating possible axial offsets of the ceramic disk, therelative contraction and expansion of the ceramic disk and pressuredeflectable diaphragm cap, when combined with the axial force providedby the spring, results in a radial tensile stress in the ceramic diskdue to frictional loading at the interface between the ceramic disk andpressure deflectable diaphragm cap. In the present embodiment, thisfrictional loading occurs where ceramic disk 252 contacts a shelf ofcounterbore 260. To reduce the possibility of cracking and/or fatiguefracture of the ceramic disk, it is desirable to include an intermediatecomponent disposed between the ceramic disk and pressure deflectablediaphragm cap. The intermediate component can act to isolate the ceramicdisk from the pressure deflectable diaphragm cap to reduce thetransferred radial stresses. As depicted in FIG. 3A, the intermediatecomponent is an annular shim 260 positioned between the ceramic disk andpressure deflectable diaphragm cap. Generally, the intermediatecomponent has a coefficient of thermal expansion that is less than thecoefficient of thermal expansion of the pressure deflectable diaphragmcap. In some embodiments, the coefficient of thermal expansion of theintermediate component is between the coefficients of thermal expansionof the ceramic disk and pressure deflectable diaphragm cap. In otherembodiments, the coefficient of thermal expansion for the intermediatecomponent is less than the coefficient of thermal expansion of theceramic disc. In yet another embodiment, the coefficient of thermalexpansion is substantially similar to the coefficient of thermalexpansion of the ceramic disk. Without wishing to be bound by theory,the reduction in the intermediate component's coefficient of thermalexpansion, as compared to the pressure deflectable diaphragm, results ina reduction of the radial tensile stresses being transferred between theceramic disk and pressure deflectable diaphragm cap. In addition toproviding a lower coefficient of thermal expansion, the intermediatecomponent can also include a coating, or be made out of, a material witha low coefficient of friction to further reduce the transferred radialstresses between the ceramic disk and pressure deflectable diaphragmcap. In other embodiments, the intermediate component can also include acoating, or is made out of, a material that is a high hardness material.Without wishing to be bound by theory, such an embodiment may help toprevent the ceramic disk from deforming the intermediate component whichwould change the gap between the opposing capacitive surfaces resultingin a zero output shift in the output sensor signal. Examples ofmaterials appropriate for use in the intermediate component includecontrolled expansion and low expansion alloys such as:iron-nickel-cobalt alloys such as ASTM alloy F-15; nickel-iron alloyssuch as ASTM alloy 52 and ASTM alloy 48; titanium and titanium alloys;and other appropriate materials as the material is not limited to theparticular alloys and materials disclosed herein.

While the intermediate component has been depicted as being separatefrom the ceramic disk and pressure deflectable diaphragm cap, theintermediate component could be embodied as a coating or surface finishapplied to either of the ceramic disk and/or pressure deflectablediaphragm cap. For example, in one embodiment, a coating having anintermediate coefficient of thermal expansion as well as a lowcoefficient of friction is applied to the surface of the pressuredeflectable diaphragm cap contacting the ceramic disk. Alternatively,the coating could be applied to the ceramic disc, or the coating couldbe applied to both the pressure deflectable diaphragm and the ceramicdisk.

In addition to providing robust components and systems capable ofcontinuously sensing pressures at elevated temperatures, it is alsodesirable to reduce the signal loss associated with the output pressuresignal to improve the sensitivity and fidelity of the signal. Since thesensitivity of the measurement is inversely proportional to the totalcapacitance of the capacitive pressure sensing circuit, it is desirableto reduce stray capacitances within the probe to reduce the totalcapacitance of the capacitive pressure sensing circuit. One way in whichto reduce signal loss is to reduce the shunt capacitance of the lead 302relative to the walls of the rigid probe body 300 as shown in FIG. 4.The shunt capacitance between the lead and rigid probe body is minimizedby maintaining the lead approximately in the center of the rigid probebody for substantially the entire length of the lead. Thus, an annularair gap 310 isolates the lead 302 from the inner wall of the rigid probebody 300. Due to the low dielectric constant of air, the resulting shuntcapacitance between the lead and rigid probe body is reduced as comparedto an annular gap comprising a solid material with a larger dielectricconstant. In addition to arranging the lead concentrically within therigid probe body, it is desirable to minimize lateral vibrations of thelead to minimize the excursions of the lead from the concentricposition. Consequently, it is desirable that the lead have lateralfrequencies of vibration above a preselected minimum as determined for aparticular application such that the first natural frequency ofvibration in the lateral direction is substantially above the expectedvibration frequencies in the intended application.

In addition to the primary bushing discussed above, as depicted in FIG.4, in one embodiment, lead 302 is also substantially concentricallylocated in the rigid probe body 300 through the use of supports 304supporting the lead 302. However, in embodiments where a secondary tube,such as a shielding tube or temperature sensor securing tube, is usedwithin the rigid probe body, supports 304 are located within thesecondary tube. In the depicted embodiment, the supports are disk-shapedcomponents that have an outer circumference 306 that is captured withinthe bore of the rigid probe body 300. The depicted supports 304 alsoinclude a hole 308 through which the lead passes and is supported. Theplurality of supports are intermittently spaced along the rigid probebody to minimize the static deflection of the lead. Further, the spacingbetween the supports is selected to provide natural frequencies oflateral vibration above a preselected minimum vibration frequency. Inorder to avoid unnecessary increases in the shunt capacitance betweenthe lead and rigid probe body, it is desirable fabricate the supportsfrom a relatively low dielectric constant material. In addition, toprevent the supports from thermally expanding inward and possiblycompressing the lead and locking it in place relative to the rigid probebody, it is desirable that the supports be made from a relatively lowcoefficient of thermal expansion material. Suitable materials include,but are not limited to, quartz, ceramic, and fused silica.

In an alternative embodiment, the shunt capacitance between the lead 302and rigid probe body 300 is reduced through the use of a shieldingsleeve located between the lead and wall of the rigid probe body. In theshielded configuration, it is less critical to maintain the leadperfectly centered within the shielding sleeve.

In some embodiments, it is desirable to include a temperature sensor inthe distal pressure sensing end of the probe to measure the processtemperature to allow for temperature compensation of the signal and/oroutput of the process temperature. Appropriate temperature sensorsinclude, but are not limited to, thermocouples, thermistors, and otherappropriate temperature sensing devices. However, regardless of thespecific temperature sensing device, temperature sensor leads wouldnecessarily need to traverse the length of the probe body to output thetemperature. The presence of these temperature sensor leads, and/ortheir metal sheaths, result in capacitive leakage from the capacitivepressure sensor lead. Further, any movement of the temperature sensorleads within the probe body would alter the capacitance between thecapacitive pressure sensor lead and the temperature sensor leads,resulting in undesirable shifts in the capacitive pressure signaloutput.

In one embodiment, the temperature sensor leads are distanced from thelead 402 to reduce the capacitive leakage. As depicted in FIGS. 5-5B, achannel is formed in the wall of the rigid probe body 400 extending fromthe proximal to distal end thereof. The temperature sensor leads arepositioned in a channel. The channel can be a groove 404, a bore 405, orany other appropriate feature formed in the wall of the rigid probe bodycapable of retaining the temperature sensor leads. Since the channel isformed in the wall of the rigid probe body, the temperature sensor leadsare positioned further away from the lead 402 than if they were locatedwithin the interior portion of the probe body bore. This increaseddistance results in a reduced capacitance between the temperature sensorleads and lead 402. In addition, as discussed in more detail below, lead402 can be shielded from the temperature sensor leads disposed of in thechannel. The channel is formed in the rigid probe body 400 using anyappropriate method including, but not limited to, electrical dischargemachining, grinding, or other appropriate machining process. To permitoutput of the temperature signal, a corresponding channel 406 is formedin a base portion 414 (FIG. 5A) of the probe body and a cutout 408 isformed in bushing 416. These features, channel 406 and outlet 408, aresubstantially aligned with the groove 404. As depicted in the figure,and discussed in more detail above, the bushing 416 is a thin quartzdisk, or other appropriate material, located within the base portion 414of the probe body to substantially concentrically support the lead inthe center of the probe body.

To prevent shifts in the output capacitive pressure signal, it isdesirable to ensure that the temperature sensors are retained in thechannel. Consequently, in some embodiments, the temperature sensor leadsare brazed, welded, or attached to the channel using other appropriateattachment methods. However, it should be noted that restraining axialmovement of the temperature sensor leads at one or more points withinthe channel could lead to increased stresses due to a mismatch in thecoefficients of thermal expansion of the temperature sensor leads andthe rigid probe body. Thus, in some embodiments the temperature sensorleads are permitted to axially slide within the channel. In one suchembodiment, as shown in FIG. 5A-5B including a groove 404, a temperaturesensor securing tube 410 is provided within the internal bore of therigid probe body to retain the temperature sensor leads within groove404. In addition to retaining the temperature sensor leads within groove404, temperature sensor securing tube 410 can also act to shield lead402 to further reduce the shunt capacitance in addition to the reductionin shunt capacitance from moving the temperature sensor leads outsidethe main portion of the probe body bore. As shown in the figure, thetemperature sensor securing tube 410 is axially aligned with and pressfit into the central bore of the rigid probe body 400, holding thetemperature sensor leads in the channel and outside the outer diameterof the temperature sensor security tube. Alternatively, the temperaturesensor securing tube could be secured in position using brazing,welding, and arrangements, fasteners, or any other appropriate method.Regardless of the specific attachment method, the, temperature sensorsecuring tube 410 is substantially maintained in a concentricarrangement with lead 402 while maintaining the temperature sensor leadswithin groove 404. FIG. 6 shows a schematic representation of atemperature sensor probe 412 positioned in groove 404 and retained bytemperature sensor securing tube 410. As described above, thetemperature sensor probe 412 is output through bushing 416.

FIG. 7 presents an exemplary graph of a capacitive pressure sensoroutput versus pressure for two different temperatures. In the depictedgraph, the lower temperature corresponds to line 500 and the highertemperature corresponds to line 502. Due to changes in part geometry andmaterial properties from thermal expansion and changes in dielectricconstant versus temperature, the high and low temperature pressuresensor outputs differ from one another. Shift 504 is the zero outputshift of the pressure sensor corresponding to the change in the outputsignal at zero pressure when the temperature is raised from lowtemperature 500 to the higher temperature 502. Without wishing to bebound by theory, the shift in the zero output is due to changes in thecapacitance of the pressure sensor versus temperature. As shown in thefigure, the difference in the output signal between the differenttemperatures at the maximum applied pressure is attributed to more thanjust the shift in the zero output signal. This is due to a shift 508 inthe sensor span at the different temperatures. It should be noted, thatthe sensor span at a particular temperature generally corresponds to thefull-scale output at maximum pressure minus the zero output at thattemperature. Without wishing to be bound by theory, changes in the spanare believed to correspond to the modulus of elasticity of the pressuredeflectable diaphragm decreasing with increasing temperature. This leadsto increased deflection of the pressure deflectable diaphragm at highertemperatures resulting in an increased change in the pressure sensorsignal for a given pressure.

It should be understood that the above presented graph is only aschematic representation for illustrative purposes regarding thetemperature induced changes for a single pressure sensor arrangement.Consequently, the expected change in sensor output versus pressure fordifferent temperatures will vary for different pressure sensorarrangements. Therefore, the current disclosure includes embodimentsexhibiting changes in the pressure sensor output versus temperature thatare different from that disclosed in FIG. 7.

In order to output a corrected pressure signal, it is desirable tocompensate for both the zero output shift 504 and the span shift 508 atany given temperature. While it is possible to compensate for eithereffect through the use of a temperature sensor and associatedtemperature correction algorithms at the main circuit, in someembodiments it is desirable to compensate for one or both of the zerooutput shift and span shift through component design and materialselection as described in more detail below. Thus, the pressure sensorcan intrinsically compensate for shifts in pressure sensor performanceversus temperature without the need to sense the operating temperaturesfor use in signal correction by external circuitry. Consequently, insome embodiments, it is possible to eliminate the need for a distallylocated temperature sensor which helps to eliminate at least one sourceof signal loss (i.e. stray capacitive losses between the capacitivepressure sensor lead and temperature sensor leads as described above) inaddition to overall reduction in complexity of the pressure sensor.

As shown in FIG. 8, the capacitive pressure sensor 600 includes a sensorcapacitance 602 that varies with pressure induced deflections of theelectrode on the pressure deflectable diaphragm toward the ceramic diskas described above in more detail. Further, the capacitive pressuresensor 600 also includes shunt capacitances 604 a and 604 b. Shuntcapacitance 604 a corresponds to the capacitance between the componentsof the pressure sensor and the probe body through the relatively rigidcomponent corresponding to the ceramic disc. Shunt capacitance 604 bcorresponds to the capacitance between the lead and probe body throughthe annular gap of the probe body. The total sensed capacitance 606 ofthe pressure sensor is a parallel combination of the sensor capacitance602 and the shunt capacitances 604 a and 604 b. Therefore, the outputpressure signal is altered by changes in either the sensor capacitance602 or the shunt capacitances 604 a and 604 b.

Since the pressure signal can be altered by changes in either the sensorcapacitance or the shunt capacitance, in one embodiment, the shift inzero output due to alterations in the sensor capacitance versustemperature is compensated for by providing an offsetting change in theshunt capacitances versus temperature. More specifically, in someembodiments, the change in the shunt capacitance 604 is approximatelyequal and opposite to the change in the sensor capacitance 602. Forexample, as the sensor capacitance decreases with increasingtemperature, the shunt capacitance increases with increasingtemperature. Thus, the total sensed capacitance 606 is approximately thesame over a predetermined temperature range without the need tocompensate for the temperature change, such as by using a temperaturesensor and corresponding circuitry.

As noted above, the shift in zero output of the capacitive pressuresensor is due to changes in the capacitance of the pressure sensor. Thiseffect is described in relation to FIG. 9, showing a distal, or sensingend, of a pressure sensor. As depicted in FIG. 9, the pressure sensor700 includes a pressure deflectable diaphragm cap 702 and a relativelynondeformable component, in this example a ceramic disk 704. Thepressure deflectable diaphragm cap 702 includes a diaphragm 703 andceramic disk 704 include respective first and second capacitive surfaces708 a and 708 b corresponding to opposing electrodes that define thecapacitive sensor. The ceramic disk includes a body 706 a with a length“L”. The ceramic disk also includes a protrusion 706 b extending from aledge 707 of the ceramic disc. The protrusion has a length “P”. Thesecond capacitive surface 708 b is disposed on protrusion 706 b suchthat it is spaced from the first capacitive surface 708 a by a gap 710with a length “g”. The ledge 707 on ceramic disk body 706 a is spacedfrom the capacitive surface 708 a by a distance “gl” corresponding tothe length of the counterbore 712 from shoulder 714 to the firstcapacitive surface 708 a. Without wishing to be bound by theory, the gapbetween the first and second capacitive surfaces increases withincreasing temperature due to protrusion 706 b expanding at a slowerrate as compared to the counterbore 712. This is due to the ceramic diskhaving a lower coefficient of thermal expansion as compared to thepressure deflectable diaphragm cap. It is noted that, the sensorcapacitance is inversely related to the gap length “g”. Therefore, asthe temperature increases, the sensor gap increases leading to the noteddecrease in the pressure sensor capacitance.

In one embodiment, the rate at which the gap increases is controlled bymodifying the ceramic disk protrusion length “P”, the length of thecounter bore “gl” on the pressure deflectable diaphragm cap, andselecting materials for each with specific coefficients of thermalexpansion. Such an effect is illustrated in FIG. 10 which depicts agraph of the change in gap “g” versus increasing temperature fordifferent initial protrusion lengths of 0.045 inch (800), 0.040 inch(802), and 0.035 inch (804). In the depicted examples, “gl” is alsovaried relative to “P” such that each one has approximately the sameinitial gap at 25° C. As shown in the figure, the rate of change in gap“g” is related to the initial protrusion length such that increasing theinitial protrusion length corresponds to increasing rates of change inthe gap versus temperature.

While it is possible to control the rate at which the pressure sensorcapacitance changes, as noted above, it is desirable that the decreasein the pressure sensor capacitance be compensated for by a correspondingincrease in the shunt capacitance. In one embodiment, the ceramic diskis made of a material that has a dielectric constant that increases withtemperature. Therefore, the shunt capacitance associated with theportion of the pressure sensor including the ceramic disk increases withincreasing temperature. In such an embodiment, the length of the ceramicdisk body, the thermal coefficient of dielectric constant of the ceramicdisk body, and the rate at which the gap increases are selected suchthat the total sensed signal is approximately the same over apredetermined temperature range without the need for a temperaturemeasurement to compensate for that temperature change. In one instance,the predetermined temperature range is approximately from −40° C. toapproximately 400° C. Further, the thermal coefficient of dielectricconstant for the ceramic disk can be selected and/or engineered to beany appropriate value to balance the expected variation in the sensorcapacitance versus temperature. Thus, by selecting appropriatecombinations of initial gap length, pressure deflectable diaphragm capgeometry (such as the counterbore depth), ceramic disk body length aswell as material properties such as coefficients of thermal expansionand thermal coefficients of dielectric constant, it is possible toprovide intrinsic compensation of the pressure sensor for changes in thezero output.

FIG. 11 depicts a simplified example of determining an appropriate bodylength L for the ceramic disk to compensate for the zero output changeof the pressure sensor. The ceramic disk has a thermal coefficient ofdielectric constant of 180 ppm/° C. The figure depicts an extrapolatedline 900 corresponding to the percent change in the total sensed signalover a temperature range of 25° C. to 350° C. for various lengths of aceramic disk body. As shown in the figure, the body length L isincreased until the change in the shunt capacitance versus temperatureis able to compensate for the changes in the sensor capacitance versustemperature. At this point, the two effects cancel each other out and asubstantially constant total sensed signal is output. The point at whichthis occurs in the current example corresponds to a body length ofapproximately 0.375 inches.

While the above description and example are directed to compensating forchanges in the zero output versus temperature, as previously noted, itis also desirable to compensate for changes in the capacitive pressuresensor span. Several nonlimiting examples of parameters which affect thecapacitive pressure sensor span include, but are not limited to, changesin the gap length due to thermal expansion and changes in the pressuredeflectable diaphragm's modulus of elasticity versus temperature. Anexample of these parameters and their competing effects on thecapacitive pressure sensor span is depicted in FIG. 12. The graphpresents the output capacitive pressure sensor signal versus the gaplength between the capacitive surfaces of the pressure sensor. Pointslocated towards the right of the graph correspond to increasing initialgap lengths. These initial gap lengths are then compressed by apreselected deflection corresponding to a pressure being applied to thepressure deflectable diaphragm.

The graph depicts the output signals from a pressure sensor at a lowerfirst temperature 1000 and a higher second temperature 1002. Thepressure sensor corresponding to the lower temperature 1000 includes aninitial gap length Z1 at zero pressure (and corresponding outputcapacitance) and an initial full-scale output FS1 when subjected to themaximum deflection. The output signal of the pressure sensor at thelower temperature 1000 has a first span S1. If the initial gap length isZ2 for the pressure sensor at the higher temperature 1002, whichcorresponds to the same initial gap length Z1 as the lower temperature,the pressure sensor will have a larger full-scale output FS2 at themaximum deflection and a correspondingly larger second span S2 ascompared to the pressure sensor at a lower temperature. This change inthe span alters the relation between the pressure and output signal.Therefore, it is desirable to compensate for this span change.

Without wishing to be bound by theory, due to the nonlinear nature ofthe pressure sensing capacitance, the span is strongly influenced by theinitial gap length. More specifically, due to the output signal beinginversely proportional to the gap length between the two capacitivesurfaces, the pressure sensor output signal exhibits a decreasing slopewith increasing initial gap lengths. Therefore, larger initial gaplengths correspond to smaller pressure sensor spans for a given amountof deflection. This effect can be combined with the altered pressuresensor performance at higher temperatures to control the shift in thespan of the pressure sensor. For example, when operating at the highertemperature 1002, if the initial gap length is shifted to a largerinitial gap length Z3, the full-scale output FS3 at this highertemperature is reduced thus resulting in a reduction in a correspondingthird span S3 as compared to span S2. In some embodiments, the initialgap length Z3 at the higher temperature is selected such that the thirdspan S3 is substantially the same as the first span S1. Consequently, insuch an embodiment, the capacitive pressure sensor output span issubstantially constant over a predetermined temperature range. Forexample, the capacitive pressure sensor output span can be substantiallyconstant from approximately 25° C. to approximately 350° C. The decreasein span versus increasing gap length is illustrated in Table I belowwhich was calculated assuming a fixed maximum pressure deflectablediaphragm displacement of 0.00075 inches for an exemplary pressuresensor.

TABLE I Zero Span Gap (pF) (pF) .0015 1.69 0.41 .00175 1.6 0.39 .002 1.50.32

In view of the above, in one embodiment, the increase in the capacitivepressure sensor span due to a reduction in the modulus of elasticity ofthe diaphragm is offset by a corresponding increase in the gap lengthwhich acts to correspondingly decrease the capacitive pressure sensorspan. As described with regards to FIG. 7, the gap length changes versustemperature due to a mismatch between the coefficients of thermalexpansion of the pressure deflectable diaphragm and the correspondingceramic disk holding the opposing capacitive surfaces that form thecapacitive pressure sensor. Therefore, in order to offset the notedincrease in the span due to changes in the modulus of elasticity versustemperature, it is desirable to provide a corresponding controlled rateof change of the gap length versus temperature. The rate of change ofthe gap length versus temperature can be controlled, as explained, byaltering the geometry of the components as well as selecting materialswith specific coefficients of thermal expansion.

In an embodiment similar to that shown in FIG. 9, the length of theprotrusion protruding into the diaphragm expands in an axial directionat a slower rate than the corresponding counterbore of the pressuredeflectable diaphragm cap. This is due to the ceramic disk having alower coefficient of thermal expansion as compared to the pressuredeflectable diaphragm cap. It should be noted that larger counterborelengths in the pressure deflectable diaphragm cap combined with largerprotrusion lengths on the ceramic disk result in increased rates ofgrowth of the gap length at higher temperatures. Thus, the protrusionlength of the ceramic disk and the corresponding counterbore length inthe pressure deflectable diaphragm cap can be selected to provide aspecific rate of change in gap length versus temperature tosubstantially offset the span increases associated with changes in themodulus of elasticity. More generally, it is possible to offset theincrease in span versus temperature by altering component geometry andselecting specific coefficients of thermal expansion of the componentsin the pressure sensor to control the rate at which the gap opens orincreases with increasing temperature. It should be understood that thespecific relation between the protrusion length and counterbore lengthchanges for different combinations of coefficients of thermal expansion.

In one specific embodiment, the pressure deflectable diaphragm is madefrom nickel-chromium based alloys such as UNS N07718 with a modulus ofelasticity that decreases at a rate of approximately −2.6% per 100° C.In such an embodiment, the span therefore will increase at a rate ofapproximately 2.6%/100° C. Therefore, the specific geometry andcoefficients of thermal expansion of the components in the capacitivepressure sensor are selected to provide a corresponding decrease in thespan of approximately −2.6%/100° C. due to a preselected rate ofincrease in the gap length versus temperature.

While described separately, it should be understood that the abovedisclosed methods for correcting the variations in zero output andpressure sensor span can be incorporated into a single pressure sensor.Thus, a pressure sensor can be provided that intrinsically corrects forboth variations in zero output and pressure sensor span without the needfor an additional temperature sensor.

EXAMPLE

One exemplary embodiment of a portion of the pressure sensor is depictedin FIG. 13. In the depicted embodiment, the pressure sensor includes agap “g” between the capacitive surfaces 708 a and 708 b that is betweenapproximately 0.0015 inches to approximately 0.0020 inches. Theelectrode present on the second capacitive surface 708 b, not depicted,corresponds to a metallization layer with a total thickness betweenapproximately 0.0006 inches to approximately 0.0013 inches. Themetallization layer comprises a molybdenum-manganese metallization layerthat is fired to adhere it to the underlying ceramic. An alloy of nickeland gold is subsequently plated onto the molybdenum-manganese substrateto form the overall metallization layer that forms the electrode. Thecounterbore 712 of the pressure flexible diaphragm has a depth “g1”between approximately 0.035 inches to approximately 0.045 inches. Thelength of ceramic disk body 706 a, as described above, corresponds totwo separate lengths “L₁” corresponding to the full diameter portion ofthe ceramic disk and “L₂” corresponding to a reduced diameter portion onthe proximal end of the ceramic disk to accommodate the biasing spring.In the depicted embodiment, “L₁” is between approximately 0.040 inchesto approximately 0.060 inches and “L₂” is between approximately 0.040inches to approximately 0.300 inches. In addition to the abovecomponents and dimensions, an intermediate component 716, is included inthe depicted embodiment. The intermediate component has a thickness “s”between approximately 0 inches to approximately 0.005 inches. Athickness of 0 inches corresponds to an embodiment that does not includean intermediate component. While the above dimensions are given ininches, they are easily converted into SI units using a conversionfactor of 2.54 cm per inch. Further, it should be understood that whileexemplary values for the components dimensions have been given, otherconfigurations of the disclosed pressure sensor with differentdimensions and arrangements of components are also possible.

While the present teachings have been described in conjunction withvarious embodiments and examples, it is not intended that the presentteachings be limited to such embodiments or examples. On the contrary,the present teachings encompass various alternatives, modifications, andequivalents, as will be appreciated by those of skill in the art.Accordingly, the foregoing description and drawings are by way ofexample only.

What is claimed is:
 1. A pressure sensor, comprising: a probe body; acapacitive sensor disposed at a distal end of the probe body, thecapacitive sensor producing a sensing capacitance; a shunt capacitance;and wherein a change in the sensing capacitance resulting from a changein temperature is intrinsically offset by a corresponding change in theshunt capacitance.
 2. The pressure sensor according to claim 1, whereina capacitive sensor comprises: a pressure deflectable diaphragm caphaving a first capacitive surface electrically coupled to the probebody; a relatively non-deformable component having a second capacitivesurface coupled to the pressure deflectable diaphragm cap, the secondcapacitive surface being spaced-apart from the first capacitive surface;and a lead electrically coupled to the second capacitive surface andisolated from the probe body; wherein the shunt capacitance existsbetween the lead and the probe body.
 3. The pressure sensor according toclaim 2, wherein the pressure deflectable diaphragm cap and therelatively non-deformable component are constructed and arranged in acooperating relationship wherein a change in capacitance of thecapacitive sensor resulting from changes in temperature is offset by achange in the shunt capacitance between the lead and the probe bodythereby providing a sensed capacitance between the lead and probe bodythat is approximately the same over a predetermined temperature rangewithout compensating for sensed temperature change.
 4. The pressuresensor according to claim 3, wherein the predetermined temperature rangeis approximately −40° C. to approximately 400° C.
 5. The pressure sensoraccording to claim 3, wherein the second capacitive surface isspaced-apart from the first capacitive surface by a gap, whereinmaterials and geometric relationship of the pressure deflectablediaphragm cap and the relatively non-deformable component arepredetermined such that, as the gap increases with increasingtemperature, an increase in the sensed capacitance associated withchanges in the modulus of elasticity of the diaphragm with increasingtemperature is offset.
 6. The pressure sensor according to claim 2,wherein the relatively non-deformable component comprises a dielectricmaterial having a dielectric constant that changes with changes intemperature.
 7. The pressure sensor according to claim 6, whereinchanges in the dielectric constant with changes in temperature resultsdirectly in a change in the shunt capacitance offsetting any changes incapacitance of the capacitive sensor.
 8. The pressure sensor accordingto claim 2, wherein the second capacitive surface is spaced-apart fromthe first capacitive surface by a gap, wherein the gap increases withincreasing temperature thereby decreasing the capacitance of thecapacitive sensor.
 9. The pressure sensor according to claim 8, whereinthe pressure deflectable diaphragm cap includes diaphragm and a shoulderand wherein the relatively non-deformable component includes a ledge,wherein the shoulder and the ledge abut each other, the relativelynon-deformable component having a protrusion extending beyond the ledge,the protrusion comprising the second capacitive surface that isspaced-apart from the first capacitive surface to define the gap,wherein a rate at which the gap increases is a function of a length ofthe protrusion.
 10. The pressure sensor according to claim 9, whereinthe relatively non-deformable component comprises a material having athermal coefficient of dielectric constant of approximately 180 ppm/° C.11. The pressure sensor according to claim 10, wherein the predeterminedtemperature range is approximately −40° C. to approximately 400° C. 12.The pressure sensor according to claim 11, wherein the pressuredeflectable diaphragm cap includes diaphragm and a shoulder and whereinthe relatively non-deformable component includes a ledge, wherein theshoulder and the ledge abut each other, the relatively non-deformablecomponent having a protrusion extending beyond the ledge toward thediaphragm and a length extending opposite the diaphragm, wherein thelength is approximately 0.375 inches.
 13. The pressure sensor accordingto claim 12, wherein the second capacitive surface is spaced-apart fromthe first capacitive surface by a gap, wherein materials and geometricrelationship of the pressure deflectable diaphragm cap and therelatively non-deformable component are predetermined such that as thegap increases with increasing temperature, an increase in the sensedcapacitance associated with changes in the modulus of elasticity of thediaphragm with increasing temperature is offset.
 14. The pressure sensoraccording to claim 2, wherein the pressure deflectable diaphragm capincludes diaphragm and a shoulder and wherein the relativelynon-deformable component includes a ledge, wherein the shoulder and theledge abut each other, the relatively non-deformable component having aprotrusion extending beyond the ledge toward the diaphragm and a lengthextending opposite the diaphragm, wherein the length is approximately0.375 inches.
 15. The pressure sensor according to claim 1, wherein adecrease in capacitance of the capacitive sensor resulting from changesin temperature is offset by an equal increase in the shunt capacitance.16. The pressure sensor according to claim 1, wherein the sensingcapacitance is approximately the same over a predetermined temperaturerange without a need for a temperature measurement to compensate fortemperature changes.
 17. A method of making a pressure sensor,comprising: arranging a pressure deflectable diaphragm cap having afirst capacitive surface on a probe body; electrically coupling thefirst capacitive surface to the probe body; selecting a material havinga desired dielectric constant and forming a non-deformable componentfrom the material; forming a second capacitive surface on a portion ofthe non-deformable component; connecting a lead to the second capacitivesurface and positioning the non-deformable component within the probebody wherein a shunt capacitance is defined between the lead and theprobe body; and arranging the non-deformable component relative to thepressure deflectable diaphragm cap by spacing the second capacitivesurface away from the first capacitive surface such that the first andsecond surfaces define a capacitive sensor having a sensing capacitance,wherein a change in the sensing capacitance resulting from a change intemperature is offset by a corresponding change in the shuntcapacitance.
 18. The method according to claim 17, further comprising:forming a ledge on a body of the non-deformable component such that aprotrusion of a predetermined length is created; forming the pressuredeflectable diaphragm cap with a diaphragm and a shoulder; abutting theshoulder against the ledge where the protrusion extends beyond the ledgetoward a diaphragm of the pressure deflectable diaphragm cap.
 19. Themethod according to claim 18, wherein a sensed capacitance between thelead and probe body is approximately the same over a predeterminedtemperature range without compensating for temperature changes.
 20. Themethod according to claim 19, further comprising forming the pressuredeflectable diaphragm cap from a material and forming the pressuredeflectable diaphragm cap with a desired geometry, wherein the secondcapacitive surface is spaced-apart from the first capacitive surface bya gap, wherein as the gap increases with increasing temperature, anincrease in the sensed capacitance associated with changes in a modulusof elasticity of the diaphragm with increasing temperature is offset.